The present invention is related to the field of digital subscriber line (DSL) communications, and in particular, to loop qualification in a DSL system.
DSL communication systems typically use copper telephone lines for high speed data transmission. A major problem for DSL service providers is to sufficiently qualify the loop or probe the line prior to the deployment. A typical system of probing the line is shown in FIG. 1. This system requires a handset 110 to be attached to the telephone line at a central office (CO) location 130, and a second handset 112 to be attached to the telephone line at the customer premises equipment (CPE) location 140. Thus, conventional line probing requires human action at two points of the telephone line.
A circuit model, including the known resistance R of the voltage source and the unknown capacitance C(1) of the telephone line, is used as a model of the telephone line as shown in FIG. 2a. The capacitance C(1) is a function of the length 1 of the telephone line. The capacitance C(1) of the telephone line is measured by applying a direct current (DC) voltage pulse VDC to the telephone line, as shown in FIG. 2b, and recording the charge-up time T of the line voltage Vline, as shown in FIG. 2c. Because the charge-up time T of the line voltage Vline is a function of the line capacitance C(1), the line capacitance can be estimated from the charge-up time T. The capacitance estimate can then be used to estimate the loop length 1 of the telephone line. The loop length in a DSL system is typically between 0.2 to 8 kilometers (Km).
However, this method cannot be used to estimate the loop length of a line that is integrated with a DSL modem board for a number of reasons. For example, conventional DSL modem boards include a line transformer to isolate the line from the CO equipment and reject the common mode voltage of the line. Due to the presence of the line transformer, a DC pulse cannot be transmitted onto the line. In addition, the resistance of the simple RC circuit of FIG. 2a includes the source resistance, but ignores the line resistance. This is a reasonable approximation of the resistance when the source resistance is much larger than the line resistance. However, in conventional DSL modem boards, the output resistance of the source is typically not significantly larger than the line resistance. As such, ignoring the line resistance degrades the accuracy of the loop length estimate.
What is needed, therefore, is a technique to qualify the loop of a DSL system.
One embodiment of the present invention provides a method for performing single ended line probing to determine if a telephone line qualifies for digital subscriber line service. The method includes transmitting low frequency signals over the telephone line, receiving reflected versions of the low frequency signals from the telephone line, measuring the transfer function of the telephone line based on the transmitted low frequency signals and received reflected versions of the low frequency signals, and qualifying the telephone line based on the transfer function.